This invention relates to backlighting apparatus such as are used for backlighting Liquid Crystal Displays. Liquid Crystal Displays (LCD's) are commonly used in portable televisions, portable computers, and cellular phones to display information to the user. For the purposes of this disclosure, an LCD can be considered to act as a light valve, which allows transmission of light in one state and blocks transmission of light in a second state. When used as a high resolution information display, an LCD is typically arranged in a dot-matrix configuration with independently addressable pixels. Each individual pixel is controlled to selectively modulate light from a backlight (transmissive mode), from a reflector (reflective mode), or from a combination of the two (transflective mode). Backlighting the LCD in some form is the most popular method of providing a source of light for such displays because backlighting the LCD allows full function in low ambient light conditions and also provides for improved contrast ratios. Because conventional monochrome transmissive dot-matrix LCD's are only approximately 40% transmissive and color dot-matrix LCD's are only approximately 6% transmissive, relatively large amounts of light are necessary to provide a visible display. The backlight must provide a substantially uniform distribution of light over the entire display in order to prevent undesirable dark spots and, where the LCD is used in portable equipment, power consumption and size must be minimized.
U.S. Pat. No. 5,050,946 (the '946 patent) discusses several conventional backlight apparatus including a light curtain system, in which light from a light source passes directly through a variable density translucent blocking layer to backlight a display. The '946 patent also discusses several conventional edge-lit light guides, including a design incorporating a flat internally diffusive light guide; a flat light guide having a diffusive front surface and a specular back reflector; and a double-wedge shaped light guide having a diffusively reflective back surface. As discussed in the '946 patent, the light guide designs are generally thinner than the light curtain designs but, because of the need to turn the light ninety degrees they are also generally less efficient than a light curtain. The light curtain, on the other hand, is generally too thick to be used as a backlight in portable equipment such as computers or cellular telephones. The '946 patent addresses the efficiency problems inherent in light guide designs by proposing a light guide having a piecewise linear double wedge-shape with a preferably specular back reflector. The light source of the '946 invention is coupled to the edge of the light guide using an index-matching material such as silicon oil, epoxy, or a polymeric material.
U.S. Pat. No. 5,461,547 discloses a thin, preferably 1 millimeter thick, light guide having a plurality of v-shaped grooves extending across the back surface of the light guide. The v-shaped grooves act as quasi-specular reflectors to reflect light entering the light guide toward the front surface.
U.S. Pat. No. 5,440,197 to Gleckman (the '197 patent) discloses an LCD backlight comprising a reflective cavity having an exit aperture. The backlight approximates an integrating sphere in that the area of the aperture from which the light escapes the cavity to illuminate the LCD display is substantially less than the total surface area of the Lambertian reflecting cavity.
In each of the foregoing light guide designs, precise geometric patterns are introduced in an effort to increase light output efficiency. Yet, in each case the light emitting aperture of the light guide is substantially equal to the cross section of the light guide itself. Thus, none of the prior art light guides have any of the light enhancing characteristics of an integrating sphere. The invention disclosed in the '197 patent approximates an integrating sphere, but does so using a relatively thick housing having a hollow cavity.
The inventors of the present invention have determined that in order for dot matrix LCDs to perform adequately in portable hand-held equipment such as cellular telephones, the backlight must be no more than 2.5 millimeters thick, must be capable of providing at least 300 foot-Lamberts of illumination with a contrast ratio (defined as the difference between the light intensity at the brightest area of the display and the light intensity at the darkest area of the display) of 1.6:1 or less, and must consume no more than 400 milliwatts of power. The prior art light guides could meet the thickness requirement, but at the specified thickness would not meet the combined light output uniformity and power consumption requirements. The quasi-integrating sphere disclosed in the '197 patent could meet the light output requirements, but would not meet the thickness or power requirements.